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        <h2 id="绘图"><a class="markdownIt-Anchor" href="#绘图"></a> 绘图</h2>
<h3 id="1-qt绘图机制"><a class="markdownIt-Anchor" href="#1-qt绘图机制"></a> 1、<code>Qt</code>绘图机制</h3>
<blockquote>
<p>Qt的绘图系统允许使用相同的<code>API</code>在屏幕和其它打印设备上进行绘制。整个绘图系统基于<code>QPainter</code>，<code>QPainterDevice</code>和<code>QPaintEngine</code>三个类。</p>
<ul>
<li><code>QPainter</code>用来执行绘制的操作；</li>
<li><code>QPaintDevice</code>是一个二维空间的抽象，这个二维空间允许<code>QPainter</code>在其上面进行绘制，也就是<code>QPainter</code>工作的空间；</li>
<li><img src="/2020/08/08/%E7%BB%98%E5%9B%BE/207222.JPG" alt></li>
<li><code>QPaintEngine</code>提供了画笔（<code>QPainter</code>）在不同的设备上进行绘制的统一的接口。<code>QPaintEngine</code>类应用于<code>QPainter</code>和<code>QPaintDevice</code>之间，通常对开发人员是透明的。除非你需要自定义一个设备，否则你是不需要关心<code>QPaintEngine</code>这个类的。</li>
<li>我们可以把<code>QPainter</code>理解成画笔；把<code>QPaintDevice</code>理解成使用画笔的地方，比如纸张、屏幕等；而对于纸张、屏幕而言，肯定要使用不同的画笔绘制，为了统一使用一种画笔，我们设计了<code>QPaintEngine</code>类，这个类让不同的纸张、屏幕都能使用一种画笔。</li>
</ul>
<p>下图给出了这三个类之间的层次结构（出自<code>Qt API</code>文档）：</p>
<p><img src="/2020/08/08/%E7%BB%98%E5%9B%BE/207221.png" alt></p>
<p>上面的示意图告诉我们，Qt 的绘图系统实际上是，使用<code>QPainter</code>在<code>QPainterDevice</code>上进行绘制，它们之间使用<code>QPaintEngine</code>进行通讯（也就是翻译<code>QPainter</code>的指令）。</p>
</blockquote>
<a id="more"></a>
<h3 id="2-绘制图形"><a class="markdownIt-Anchor" href="#2-绘制图形"></a> 2、绘制图形</h3>
<blockquote>
<p>绘制过程类似于事件处理程序，需要重写画板对象的虚函数 <code>void paintEvent(QPaintEvent **event*)</code></p>
</blockquote>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//.h</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">ifndef</span> WIDGET_H</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">define</span> WIDGET_H</span></span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;QWidget&gt;</span></span></span><br><span class="line"></span><br><span class="line">QT_BEGIN_NAMESPACE</span><br><span class="line"><span class="keyword">namespace</span> Ui &#123; <span class="class"><span class="keyword">class</span> <span class="title">Widget</span>;</span> &#125;</span><br><span class="line">QT_END_NAMESPACE</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Widget</span> :</span> <span class="keyword">public</span> QWidget</span><br><span class="line">&#123;</span><br><span class="line">    Q_OBJECT</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">    Widget(QWidget *parent = <span class="literal">nullptr</span>);</span><br><span class="line">    ~Widget();</span><br><span class="line"></span><br><span class="line"><span class="keyword">protected</span>:</span><br><span class="line">    <span class="function"><span class="keyword">void</span> <span class="title">paintEvent</span><span class="params">(QPaintEvent *event)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line">    Ui::Widget *ui;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="meta">#<span class="meta-keyword">endif</span> <span class="comment">// WIDGET_H</span></span></span><br><span class="line"><span class="comment">//.cpp</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&quot;widget.h&quot;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&quot;ui_widget.h&quot;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;QPainter&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;QDebug&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;QPixmap&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;QPen&gt;</span></span></span><br><span class="line"></span><br><span class="line">Widget::Widget(QWidget *parent)</span><br><span class="line">    : QWidget(parent)</span><br><span class="line">    , ui(<span class="keyword">new</span> Ui::Widget)</span><br><span class="line">&#123;</span><br><span class="line">    ui-&gt;setupUi(<span class="keyword">this</span>);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">Widget::paintEvent</span><span class="params">(QPaintEvent *event)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="comment">//绘图一定需要在这个函数中绘图</span></span><br><span class="line">    <span class="comment">//画家</span></span><br><span class="line">    <span class="function">QPainter <span class="title">p</span><span class="params">(<span class="keyword">this</span>)</span></span>;<span class="comment">//定义一个画家，并且指定了绘图设备</span></span><br><span class="line">    p.drawPixmap(<span class="number">0</span>,<span class="number">0</span>,<span class="keyword">this</span>-&gt;<span class="built_in">width</span>(),<span class="keyword">this</span>-&gt;<span class="built_in">height</span>(),QPixmap(<span class="string">&quot;://qt_image/earth.jpg&quot;</span>));</span><br><span class="line"><span class="comment">//    p.drawLine(50,50,300,300);</span></span><br><span class="line">    QPen pen;</span><br><span class="line">    pen.setBrush(Qt::red);<span class="comment">//设置笔的填充颜色，也可以使用setcolor()</span></span><br><span class="line">    pen.setWidth(<span class="number">3</span>);<span class="comment">//设置笔的宽度</span></span><br><span class="line">    p.setPen(pen);<span class="comment">//将笔给到画家</span></span><br><span class="line">    p.drawEllipse(QRectF(<span class="number">50</span>,<span class="number">50</span>,<span class="number">100</span>,<span class="number">100</span>));<span class="comment">//画一个矩形</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<h3 id="3-更新绘图"><a class="markdownIt-Anchor" href="#3-更新绘图"></a> 3、更新绘图</h3>
<blockquote>
<p>调用this-&gt;update更新绘图</p>
</blockquote>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//.h</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">ifndef</span> WIDGET_H</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">define</span> WIDGET_H</span></span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;QWidget&gt;</span></span></span><br><span class="line"></span><br><span class="line">QT_BEGIN_NAMESPACE</span><br><span class="line"><span class="keyword">namespace</span> Ui &#123; <span class="class"><span class="keyword">class</span> <span class="title">Widget</span>;</span> &#125;</span><br><span class="line">QT_END_NAMESPACE</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Widget</span> :</span> <span class="keyword">public</span> QWidget</span><br><span class="line">&#123;</span><br><span class="line">    Q_OBJECT</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">    Widget(QWidget *parent = <span class="literal">nullptr</span>);</span><br><span class="line">    ~Widget();</span><br><span class="line"></span><br><span class="line"><span class="keyword">protected</span>:</span><br><span class="line">    <span class="function"><span class="keyword">void</span> <span class="title">paintEvent</span><span class="params">(QPaintEvent *event)</span></span>;</span><br><span class="line">    <span class="function"><span class="keyword">void</span> <span class="title">mousePressEvent</span><span class="params">(QMouseEvent *event)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line">    Ui::Widget *ui;</span><br><span class="line">    <span class="keyword">int</span> i,j;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="meta">#<span class="meta-keyword">endif</span> <span class="comment">// WIDGET_H</span></span></span><br><span class="line"></span><br><span class="line"><span class="comment">//.cpp</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&quot;widget.h&quot;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&quot;ui_widget.h&quot;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;QPainter&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;QPixmap&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;QDebug&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;QMouseEvent&gt;</span></span></span><br><span class="line"></span><br><span class="line">Widget::Widget(QWidget *parent)</span><br><span class="line">    : QWidget(parent)</span><br><span class="line">    , ui(<span class="keyword">new</span> Ui::Widget)</span><br><span class="line">&#123;</span><br><span class="line">    ui-&gt;setupUi(<span class="keyword">this</span>);</span><br><span class="line">    i=<span class="number">0</span>;j=<span class="number">0</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">Widget::paintEvent</span><span class="params">(QPaintEvent *event)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="function">QPainter <span class="title">p</span><span class="params">(<span class="keyword">this</span>)</span></span>;</span><br><span class="line">    p.drawPixmap(i,j,<span class="number">50</span>,<span class="number">50</span>,QPixmap(<span class="string">&quot;://qt_image/20072013.jpg&quot;</span>));</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">Widget::mousePressEvent</span><span class="params">(QMouseEvent *event)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    i=event-&gt;x();<span class="comment">//返回鼠标事件的横坐标</span></span><br><span class="line">    j=event-&gt;y();<span class="comment">//返回鼠标事件的纵坐标</span></span><br><span class="line">    <span class="comment">//刷新绘图</span></span><br><span class="line">    <span class="keyword">this</span>-&gt;update();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
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